In the present study, the phase equilibrium conditions of tetra-n-butylphosphonium bromide (TBPB) hydrate forming with carbon dioxide (CO2) (0.17 mole fraction)/N-2 and CO2 (0.6171 mole fraction)/N-2 gas mixtures and TBPB aqueous solution at mass fractions w = 0.05, 0.10, and 0.15 are measured in the pressure range from (0.6 to 4.3) MPa, respectively. The CO2 separation efficiencies in a two-stage separation with TBPB hydrate are measured at 277.5 K in the initial pressure range of (2.0 to 4.0) MPa. Furthermore, three quaternary salts, i.e., tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium chloride (TBAC), and tetra-n-butylphosphonium chloride (TBPC), are adopted in the first-stage separation for comparison.